In recent years, lithium ion secondary batteries are widely used for portable electronic instruments such as cell phones or notebook-size personal computers. As a cathode active material for a lithium ion secondary battery, a composite oxide of lithium with a transition metal, etc. (hereinafter referred to also as a lithium-containing composite oxide), such as LiCoO2, LiNiO2, LiNi0.8Co0.2O2 or LiMn2O4, is employed.
Further, for a lithium ion secondary battery for portable electronic instruments or vehicles, it is desired to reduce the size and weight, and it is desired to further improve the discharge capacity per unit mass or such characteristics that the discharge capacity will not substantially decrease after repeating the charge and discharge cycle (hereinafter referred to also as cycle characteristics). Further, particularly in its application to vehicles, it is desired to further improve such characteristics that the discharge capacity will not decrease when discharging is conducted at a high discharge rate (hereinafter referred to also as rate characteristics). As a method for improving such cycle characteristics and rate characteristics, it has been known to be effective to provide a covering layer on the surface of lithium-containing composite oxide particles.
Patent Document 1 discloses a method of forming a treated surface layer containing a compound represented by a chemical formula of MXOk on the surface of an active material for a lithium ion secondary battery. Here, in the formula, M is at least one member selected from the group consisting of Na, K, Mg, Ca, Sr, Ni, Co, Si, Ti, B, Al, Sn, Mn, Cr, Fe, V, Zr and combinations thereof, X is an element selected from the group consisting of P, S, W and combinations thereof, and k is a number within a range of from 2 to 4.
Further, Patent Document 2 discloses a cathode active material for a lithium ion secondary battery having a lithium compound such as Li2SO4 or Li3PO4 impregnated on the surface of a lithium-containing composite oxide. By the presence of such a lithium compound on the surface of the cathode active material, it functions as a physical barrier, whereby dissolution, into an electrolytic solution, of manganese ions in the lithium-containing composite oxide can be suppressed, and by incorporating a compound containing a bivalent metal atom (such as ZnO), it is possible to increase the valency of manganese atoms in the vicinity of the surface of the lithium-containing composite oxide, whereby elution of manganese ions can be further suppressed.
Patent Document 3 discloses a cathode active material for a lithium ion secondary battery, which comprises a cathode active material capable of absorbing/desorbing lithium, a lithium phosphate compound and Al2O3. By mixing the lithium phosphate compound and Al2O3, it is possible to improve the lithium ion conductivity and at the same time to obtain good thermal stability and a high discharge capacity and to obtain good charge and discharge cycles.